1. Field of the Invention
This invention relates to a process for the preparation of thermosetting resin emulsions. More particularly, this invention relates to a process for the preparation of a homogeneous and stable self-emulsifiable emulsion of an amphoteric polyurethane.
2. Description of the Prior Art
As conventional methods for preparing polyurethane emulsions, there are known a method comprising emulsifying and suspending a polyurethane composition in water in the presence of an emulsifier, and a method for preparing an emulsifier-free polyurethane emulsion by forming a composition of a polyurethane containing a tertiary amino group by using an active hydrogen compound containing a tertiary amino group, and quaternizing said amino group or mixing said polyurethane composition with an aqueous solution of an acid, and the like.
It is also known that a self-emulsifiable polyurethane emulsion can be obtained by reacting a composition of a polyurethane containing free primary and/or secondary hydroxyl groups and/or amino groups with a salt or a 3- to 7-membered cyclic compound containing a group capable of forming a salt on ring-opening.
Conventional methods using an emulsifier are disadvantageous because a large quantity of an emulsifier is generally employed, so that the resulting emulsion is expensive. Also, because the emulsifier destroys the emulsion on application or has bad influences on the properties of the product obtained after drying, the fields of use of emulsions prepared by these methods are extremely limited.
In the prior art methods comprising converting the tertiary amino group contained in a polyurethane composition to an acid salt or quaternizing such tertiary amino group, or the prior art methods comprising reacting free hydroxyl and/or amino groups in a polyurethane composition with a salt or 3- or 7-membered cyclic compound containing a group capable of forming a salt on ring-opening, when the molecular weight of the polyurethane is increased to such an extent that good mechanical properties can be obtained, the stability of the polyurethane emulsion is lowered and the emulsion frequently becomes heterogeneous. In contrast, in these conventional methods, when the molecular weight of the polyurethane is decreased so as to obtain a stable emulsion, the tensile strength and elongation of the resulting polyurethane resin are drastically lowered. Thus, in these conventional methods, there is inevitably observed the disadvantage that the stability of the emulsion and the mechanical properties of the resulting polyurethane resin are contradictory to each other, that is, one cannot be increased without simultaneously decreasing the other, and vice versa.
We previously invented a cationic polyurethane emulsion of the self-crosslinking type (Japanese Laid Open Pat. application No. 33141/72) as an emulsion overcoming the foregoing disadvantages involved in the conventional techniques. In general, polymer latices are used in the state of admixture with other latices, fillers, modifiers and other additives, and in order to attain good stability and compatibility in the resulting mixed latices, polymer latices differing in their ionic characteristics are chosen and mixed.
Accordingly, the development of cationic, anionic, amphoteric and nonionic polyurethane emulsions of both the self-emulsifiable type and the self-crosslinking type is of great industrial significance.
As an effective method for imparting a self-crosslinking property to a polyurethane resin, there can be mentioned a method comprising using as a chain extender for urethane prepolymers a polyalkylene-polyamine (I) having as functional groups at least two primary or secondary amino groups and a group --CH.sub.2 -CH(OH)-CH.sub.2 X', in which X' is Cl or Br, which is prepared by adding an epihalohydrin to a polyalkylene-polyamine. However, the compound (I) is readily changed with the passage of time depending on its structure, and the reaction conditions must be strictly controlled in the reaction of adding the epihalohydrin to the polyalkylene-polyamine or the reaction of chain-extending the urethane prepolymer by the compound (I). Accordingly, when urethane prepolymers having specific structural features are used, it is difficult to produce a polyurethane emulsion in a large quantity with good reproducibility. In some cases, such disadvantages as reduced stability of the polyurethane emulsion obtained according to this method or of a latex formed by mixing such polyurethane emulsion with other polymer latices is brought about.